Servo-motor systems



Novyls, 1958 G. ORLQFF 2,86%607 SERVO-MOTOR SYSTEMS Filed May 25, 1956|NVENTOR GEon'aE ORLOFF Y m 61% M w Wm ATTORNEYS United States Patent 6SERVO-MOTOR SYSTEMS Gerge 0rlo if, Gloucester, England, assignor toBritish Messier Limited, Gloucester, England, a British com- P yApplication May 23, 1956, Serial No. 586,807. Claims priority,application Great Britain May 24, 1955 4 Claims. (Cl. 121-47) Thisinvention relates to servo-motor systems, and is concerned with liquidpressure servo-motor systems of thekind comprising a cylinder, a doubleacting piston sliding in the cylinder, a servo-control valve for thecylinder, a source of liquid pressure, a low pressure reservoir, hquidpressure supply and exhaust lines connecting said source and saidreservoir with the servo-control valve, and passages connecting theservo-contol valve with each end of the hydraulic cylinder.

Hydraulic servo-systems where the piston is locked in the position towhich it is set by trapping the liquid in the hydraulic cylinder, cansuffer from certain defects, such as creep and cavitation within thehydraulic cylinder, caused by external forces acting upon the elementoperated by the piston.

An object of this invention is to provide a servo-motor system whichtends to reduce these defects and is also improved in other respects.

According to the invention there is provided a liquid pressureservo-motor system comprising in combination, a hydraulic cylinderhaving a double acting hydraulic piston movable therein, a servo-controlvalve for the cylinder, a low pressure reservoir, a liquid pressurizingand distributing means supplied from the reservoir, liquid pressuresupply and exhaust lines connecting said pressurizing means and saidreservoir with the servo-control valve, passages connecting theservo-control valve with either end of the hydraulic cylinder, and valvemeans in said passages, which valve means act independently of theservo-control valve and permit flow of liquid through said passagesunder normal operating conditions but operate to close off and isolatesimultaneously the passages upon the pressure of the liquid supplyfalling below a predetermined value.

Preferably, a valve is located in the liquid pressure supply line of theservo-control valve, which valve closes automatically when the pressureof the liquid supply drops below a predetermnied minimum value to shutoff the supply, said shut-off valve and the said isolating valve meansbeing coupled together so that closing of the one is accompanied byclosing of the other.

One embodiment of the invention is hereinafter described by way ofexample, and with reference to the accompanying diagrammatic drawing,which shows a liquid pressure servo-motor according to the invention.

Referring to the drawing, the servo-motor which is generally indicatedat comprises a cylinder 11 in which is slidable a piston 12 mounted on arod 13 which projects through both ends of the cylinder 11. The rod isof uniform diameter so that the effective areas of both faces of thepiston are equal to one another.

A servo-control valve 14, the body 15 of which is mounted on thehydraulic cylinder 11 includes a plunger 16 slidable in a bore 17 in thebody 15. The plunger is provided with two spaced circumferential grooves18 and 19 bounded by three lands 20, 21, and 22. A second bore 23 inparallel relation to bore 17 is provided in the servo control valvecasing and disposed between the bore 17 and the cylinder 11. This bore23 is increased in diameter at its centre part 23b thereby forming steps24 and 25. The left-hand part 23a of the bore 23 forms the pressureliquid inlet to the hydraulic cylinder 11. The centre and right handparts 23b and 23c, together house an isolating valve 26. This valve hasa conical portion 27 at its left-hand end and adjacent to this conicalportion is provided with a flange 28. A coil spring 29is interposedbetween this flange and the step 25. That part of the isolating valve 26housed within the right-hand parts 230 of the bore 23 is provided withtwo circumferential grooves 30 and 31 and thus with lands 32 and 33respectively to the right of each groove. These lands are a lapped fitin the bore.

A passage 34 connects the centre part 231) of bore-23 with an annulus 35surrounding the land 21 of the valve plunger 16, while passages 36 and37 respectively place the circumferential grooves 18 and 19 incommunication with the circumferential grooves 30 and 31 when the valve26 is held against the pressure of the coil spring 29 in its rightwardposition. Passages 38 and 39 respectively lead from the circumferentialgrooves 30 and 31 to the left and right-hand ends of the hydrauliccylinder 11. Annuli 40 and 41 surrounding the lands 20 and 22 of theplunger 16 are connected to a reservoir 42 through a common passage 43and conduit 44. I

Liquid under pressure for operating the piston is supplied by a liquidpressure accumulator 45 from which a conduit 46 leads to the liquidpressure inlet 23a. Pressure is maintained in the accumulator 45 by apump 47 which draws liquid from the reservoir 42. A cut-out valve 48 isprovided which acts in known manner to connect the output of the pump 47to the accumulator 45 when the pressure in the latter falls to apredetermined lower limit, and when the pressure in the accumulatorrises to a predetermined higher limit disconnects the output of the pumpfrom the accumulator and connects it to the low-pressure reservoir 42through a conduit 49 which branches into conduit 44. A non-return valve50 is provided in conduit 44 between the passage 43 and the point atwhich conduit 49 branches into conduit 44.

In operation, when the servo-control valve plunger 16 is in its neutralposition the lands 20 and 22 thereof close over the annuli 4t} and 41and thus both ends of the hydraulic cylinder 11 are isolated from theliquid return passage 43. Also the land 21 closes over the annulus 35and thus prevents pressure liquid from entering either of thecircumferential grooves 18 and 19 of the valve.

Under normal operating conditions liquid under pressure is admitted fromthe delivery conduit 46 through the inlet bore 23a and acts upon theconical end 27 of the isolating valve 26 so as to lift it from itsseating, formed by the step 24, against the effort of coil spring 29.Thus pressure liquid is permitted to pass into the passage 34 and theannulus 35 and as long as the pressure is maintained above apredetermined value the isolating valve 26 will remain open and pressurefluid will be maintained in the passage 34 and the annulus 35.

If the servo-control valve plunger 16 is moved to the right in thedrawing the land 21 opens the annulus 35 to the circumferential groove18 so that pressure liquid can pass from the annulus into thecircumferential groove 18 and therefrom through passage 36, through thecircumferential groove 30 in the isolating valve 26, and through passage38 to the left-hand side of the piston 12. Consequently the piston 12and rod 13 move to the right. Liquid on the right-hand side of thepiston is exhausted through passage 39, circumferential groove 31,passage 37, circumferential groove 19, annulus 41, passage 43 andconduit 44 to the reservoir 42.. If the servo-control valve plunger 16is moved to the left the system works in a reverse manner to move thepiston 12 and rod 13 to the left.

If however for some reason, such as for example conduit fracture,leakage or blockage, the pressure of the liquid at the inlet bore 23adrops below the aforementioned predetermined value,'- the isolatingvalve 26 moves to the left-under the effort of coil spring 29, conicalpart 27 closing on toits seating. Consequently lands 32 and 33'close'off passages 38 and 39 from passages 36 and 37. Thus the piston 12 androd 13 become positively hydraulic'ally locked with respect to thecylinder 11 and no outside force, acting upon the element which theservo-motor controls, can displace them.

I claim:

1. A liquid pressureservo-motor system comprising in combination ahydraulic cylinder having a double acting hydraulic pistonmovable-therein, a servo-control valve for the cylinder, a low pressurereservoir, liquid pressurizing and distributing means supplied from thereservoir, liquid pressure supply and exhaust lines connecting saidpressurizing means and said reservoir with the servocontrolvalve,,passages connecting the servo-control valve with either'endof'the hydraulic cylinder, and valve means in'said passages, which valvemeans act independently of the servo-control valve and permit flow ofliquid through said passagesunder normal operating conditions butoperate' to close off and isolate simultaneously the passages upon thepressure of the liquid supply falling below apredetermined value.

2. A system as claimed in claim 1 wherein a valve is located in theliquid pressure supply line of the servocontrol valve, which valvecloses automatically when the pressure of the liquid supply drops belowa predetermined minimum value to shut off the supply, said shut-off?valve and the said isolating? valve means being coupled together so thatclosing of 'the one is accompanied by closing of the other.

3. A system as claimed'in claim 2, wherein the isolating valve meanscomprises a valve spool sliding in a bore in a valve block, one end ofthe valve spool projects into a chamber in the valve block through whichthe pressure liquid from the supply line passes on'its way to theservocontrol valve, and the end of the valve spool in the chamher isformed as the shut-off valve to seat over. theliquid pressure inlet tothe chamber, the valve spool being spring urged toward said inlet butbeing held off it during'normal operation by the supply line pressure.

4. A system as' claimed in claim 3, wherein the servocontrol valve alsocomprises a valve spool sliding in a valve bore and the bores of theservo-control and isolating valves are formed parallel to one another inacommon valve block mounted on the hydraulic cylinder, the

isolating valve being positioned between the servo-control valve and thehydraulic cylinder.

References Cited in the file of this patent UNITED STATES PATENTS2,127,877 Maglott Aug. 23, 1938, 2,132,953 Indge Oct.'1l, 1-9382,403,391 Muir July 2, 1946: 2,483,312 Clay Sept. 27, 1949 2,648,346Deardorff Aug. 11', 1953- 2,738,772 Richter Mar. 20, 1956:

